DISSOLVED AIR FLOATATION (DAF) THICKENERS

Dissolved air floatation thickening relies on a process similar to what happens to sludge during denitrification. Small air bubbles attach themselves to sludge particles and separate them from the remaining liquid by floating them to the surface where they accumulate into a sludge layer. This sludge layer is skimmed off into a hopper as thickened sludge. A large portion of the separated water is recycled to the feed sludge line after being saturated with air (this provides the bubbles for floatation). Effluent exits the far end of the unit and is returned to the treatment plant flow
stream. Polymers are often used to enhance the separation and thickening characteristics of the feed sludge.

The performance of DAF units depends upon the following factors:

1. Type of sludge. Primary sludges are less adaptable to floatation than secondary sludges. Sludge age affects how well secondary sludge thickens. “Young” sludge thickens better than “old” sludge. Primary sludges tend to deposit grit and sediment in the bottom of DAF units so provisions must be made for removal of this material.
2. Air to solids ratio. The amount of air available to float the sludge is critical to the thickening operation. The air pressure in the saturation tank determines the quantity of air actually saturated into the recycle stream.
3. Recycle rate. The recycle flow carries the saturated air to the sludge feed inlet in the DAF tank. When the recycle stream goes from a pressurized condition to atmospheric pressure, numerous small bubbles form as the air comes out of solution (similar to opening a can of soda). Obviously, the higher the recycle rate the more air that is available for floatation. However, the air saturation ratio and the recycle rate are dependent upon one another.
4. Thickness of floating sludge blanket. Adjusting the speed of the surface sludge scrapers can vary the thickness of the floating sludge blanket. Increasing the scraper speed tends to thin out the floated sludge. Conversely, decreasing the scraper speed will tend to result in a thicker sludge. However, the sludge removal rate must be fast enough to prevent solids from carrying over into the effluent. The floating sludge blanket is usually 6 – 8 inches deep. DAF units should produce an effluent that has less than 100 mg/L TSS when operating properly.